The present invention relates to a process for the manufacture of bis(4-hydroxyaryl)alkanes by heterogeneous acid-catalyzed reaction of aromatic hydroxy compounds with ketones in reactors connected in series, which are operated with rising temperature and optionally with increasing loading in the direction of advancing conversion.
It is known in a process for the manufacture of bis(4-hydroxyaryl)alkanes from phenols and ketones to distribute the required amount of ketone to several reactors connected in series. U.S. Pat. No. 2,775,620 describes a process catalyzed with mineral acid in the homogeneous liquid phase; a heterogeneously catalyzed process with an acid ion exchanger in a fixed bed reactor follows from U.S. Pat. No. 4,400,555 (EP-A 342 758). Both documents show that by dividing up the amount of ketone the proportion of by-products is reduced, this proportion being less in the case of HCl catalysis than in the case of catalysis by ion exchangers. This is only demonstrated however, for an educt mixture of phenol and acetone. In a continuously operated production plant, however, the mother liquor obtained after separation of the bis(4-(hydroxyaryl)alkane from the reaction mixture is as a rule returned to the process. In the course of this, isomers and by-products can concentrate, which leads to disturbances in the process, e.g. worse crystallization of the bis(4-hydroxyaryl)alkane and lower product quality.
It is therefore desirable to develop processes in which under the conditions of a continuously operated plant the lowest possible proportions of isomers and by-products are formed.
A process has now been discovered for the manufacture of bis(4-(hydroxyaryl)alkanes by heterogeneous acid-catalyzed reaction of aromatic hydroxy compounds with ketones in at least two fixed bed reactors connected in series which are operated with temperature rising in the direction of advancing conversion,. wherein the total amount of ketone is divided up over the individual reactors and is distributed homogeneously in the reaction mixture before the particular catalyst beds are entered.
Suitable aromatic hydroxy compounds for the process according to the invention are not substituted in the p position and contain no second-order substituents such as cyano, carboxy or nitro groups; there may be mentioned for example phenol, o- and m-cresol, 2,6-dimethylphenol, o-tert-butylphenol, 2-methyl-6-tert-butylphenol, o-cyclohexylphenol,o-phenylphenol,o-isopropylphenol,2-methyl-6-cyclopentylphenol, o- and m-chlorophenol and 2,3,6-trimethylphenol. Phenol, o- and m-cresol, 2,6-dimethylphenol, o-tert-butylphenol and o-phenylphenol are preferred; most preferred is phenol.
Suitable ketones contain at least one aliphatic group on the carbonyl function; there may be mentioned for example acetone, methyl ethyl ketone, methyl propyl ketone, methyl isopropyl ketone, diethyl ketone, acetophenone, cyclohexanone, cyclopentanone, methyl-, dimethyl- and trimethylcyclohexanone, which also can have geminal methyl groups, like 3,3-dimethyl-5-methylcyclohexanone (hydroisophorone). Acetone, acetophenone, cyclohexanone and its homologues bearing methyl groups are preferred; most preferred is acetone.
Preferred educts for the process according to the invention are particularly also the mother liquors remaining after separation of the bis(4-hydroxyaryl)alkanes, which, after addition of the hydroxy compounds consumed and optionally removal of a certain proportion to avoid the enrichment of unwanted by-products, are returned to the process; in the case of the synthesis of bisphenol A such mother liquors contain about 78-88 wt. % phenol and 12-22 wt. % bisphenol A and by-products, which have the following composition:
The molar ratio of aromatic hydroxy compound to ketone is in general 5:1 to 25:1, preferably 7:1 to 20:1, most preferably 8:1 to 18:1, relative to the overall reaction.
The educt mixture used can contain small amounts of water, preferably less than 1, most preferably less than 0.6 and especially less than 0.3 wt. %.
The ion exchanger resins used as catalysts and the mercapto compounds used as cocatalysts are well known to the person skilled in the art (U.S. Pat. Nos. 2,468,982; 2,623,908; 2,775,620; DE-OS 3 619 450; 3 727 641).
In continuous operation, before each reaction cycle the amount of aromatic hydroxy compound consumed in the preceding cycle is readded to the reaction mixture. The amount of ketone required for the adjustment to the required molar ratio of hydroxy compound and ketone is divided up over the n reactors of the production plant, approximately the nth part of the total amount of ketone being added to the reaction mixture before each reactor. The deviation from this value for the individual reactors can be absolutely xc2x115%, preferably xc2x110% and most preferably xc2x15 %.
The number of reactors is at least 2 and for economic reasons is as a rule not more than 8, preferably not more than 6, most preferably not more than 4.
The loading, defined as the amount (in kg) of educt mixture per liter catalyst in the operating condition (swollen) and per hour, is about 0.1 to 2.0 per reactor, preferably 0.15 to 1.7, most preferably 0.19 to 1.5 kg/lxc2x7h. The loading should as a rule be so chosen that the conversion of acetone after the last reactor is at least 75%, better xe2x89xa783%, preferably xe2x89xa790% and most preferably xe2x89xa795%.
It is not necessary to operate all reactors with the same loading. Rather, it is advantageous for a further increase in selectivity to raise the loading of the reactors from reactor to reactor in the direction of increasing conversion. For example in a plant with three reactors, the first reactor can be operated with 0.3, the second reactor with 0.6 and the third reactor with 0.8 kg/lxc2x7h.
For an effective reduction of the amount of by-product it is very important that before a particular catalyst bed is entered the ketone is distributed completely homogeneously in the reaction mixture, which can be achieved by the use of nozzles, static mixers, stirred tanks, centrifugal pumps or other mixing apparatuses familiar to the person skilled in the art.
The reactors connected in series are operated with temperatures rising in the direction of advancing conversion. Between start and end of the reactor cascade a rising temperature profile is set in the temperature range of 40 to 100xc2x0 C., preferably 45 to 90xc2x0 C., most preferably 50 to 85xc2x0 C. The temperature differences from one reactor to the next are as a general rule the smaller the more reactors have to be passed through. It is also possible to operate two successive reactors with the same temperature.
Since no intermixing occurs in fixed bed reactors and dissipation of the heat of reaction from the reaction mixture is difficult, such reactors are as a rule operated adiabatically, which leads to the heating of the reaction mixture. It is therefore usually expedient to cool the reaction mixture between the individual reactors, care having to be taken that the crystallizing-out of bis(4-hydroxyaryl)alkane, which would lead to a blockage of the tubing, is avoided.